Understanding monitor/printer profiles?

You can take one single file (with whatever settings your prefer), and send that same file and settings to Costco once a week for 4 weeks and you will get back 4 prints with differences.  I have done this with top labs MPIX and White House Custom Color (WHCC) and the same thing happens.  The differences are minor but they will be there, such as a blue sky will look just great on one print and then a slightly different hue on the next one.

Bob P.